AC power is now universally adopted all over the word because of its easy conversion from one voltage level to another using energy-efficient power transformers.
From a large ac power system (power grid), large power users are catered to at high voltage, medium power users at medium voltage, and low power users at a safe low voltage of 120 or 240 V.
In a single-phase (1-ph) power circuit, the instantaneous power varies between the maximum and minimum values every cycle, with the average power positive.
In a balanced three-phase (3-ph) power circuit, when the instantaneous power in one phase is decreasing, the power in the remaining phases is increasing, making the sum of power in all three phases a steady constant value with no time variations.
It is for this smooth power flow that the 3-phase ac system has been universally adopted around the world. Since all three phases are balanced and identical, the balanced 3-phase power circuit is generally shown by a 1-line diagram, and analyzed on single-phase (per phase) basis.
Then, the Average 3-phase power = 3 × Average 1-phase power.
The 3-phase source or load can be connected in Y or Δ. The basic voltage and current relations between the line to neutral (called phase) values and the line to line (called line) values are now reviewed.